Electric motors are used in a variety of electrical equipment. For example, linear electric motors produce electrical power propelling an armature in one dimension. Wafer stages utilize linear electric motors to position a wafer during photolithography and other semiconductor processing.
Electric motors are used in a variety of electrical equipment. For example, linear electric motors produce electrical power propelling an armature in one dimension. Wafer stages utilize linear electric motors to position a wafer during photolithography and other semiconductor processing.
A typical one-dimensional linear electric motor has a magnet track with pairs of opposing magnets facing each other. (U.S. Pat. No. 4,151,447, Linear Motor, issued to Johann von der Heide and Georg F. Papst on Apr. 24, 1979 discusses one-dimensional linear electric motors and is incorporated by reference herein in its entirety.) Within spaces between the pairs of opposing magnets, an armature moves. The armature has windings of a conductor which are connected to an electrical current. When the electrical current is turned on, the electrical current interacts with the magnetic fields of the magnet pairs to create force on the armature. When the armature is attached to a wafer stage, the wafer stage experiences the same force which can be used to cause movement.
In a multiphase motor, the armature has various windings grouped into phases. The phase groups are selectively pulsed with electric current to create a more efficient motor. As the armature moves within the magnet track as a first group of coils is pulsed, the first group moves out of its optimal position between the pairs of magnets. Then, it becomes more efficient to pulse a second group of windings. More phase groups are theoretically more efficient since a more even application of force and utilization of power input is maintained. However, each additional phase group complicates a timing of the pulses to the various phase groups. Presently, three-phase motors and armatures have gained favor in balancing these considerations.
Linear two-dimensional motors are also used in manufacturing. (U.S. Pat. No. 4,654,571, Single Plane Orthogonally Moveable Drive System issued to Walter E. Hinds on Mar. 31, 1987 (Hinds) and U.S. Pat. No. 4,535,278, Two-Dimensional Precise Positioning Device for Use in a Semiconductor Manufacturing Apparatus issued to Teruo Asakawa on Aug. 13, 1985 (Asakawa) discuss two-dimensional linear electric motors and are incorporated by reference herein in their entirety.) The motors are two-dimensional in that they have two-dimensional arrays of magnets and armatures instead of magnet tracks and one-dimensional armatures. However, the magnet arrays and two-dimensional armatures may move with respect to each other in more than two dimensions depending upon the design. Conventional two-dimensional linear motors typically have an array of magnets and an armature having one or more coils on one side of the array of magnets. When attached to part of a two-dimensional linear motor, a platform can be moved in two or more dimensions by the motor. For example, a wafer stage in semiconductor processing equipment may be attached to an armature or magnet array of a two-dimensional motor, and the two-dimensional motor would control positioning of the wafer stage.
A problem with conventional magnet arrays is their relatively low magnetic flux-to-mass ratio. Without a backing by a magnetically permeable material, the flux is fairly low. A magnetically permeable backing permits completed flux paths between magnets having the same polarity. Of course, the flux of the array is greater when the flux paths are completed by the magnetically permeable backing. However, magnetically permeable materials, such as iron, are relatively heavy. If the flux paths between the magnets of the same polarity could still be completed without using a heavy magnetically permeable backing, the magnetic flux-to-mass or weight ratio would be improved. For electric motors having moving magnets instead of moving coils, this reduction in mass or weight would improve the efficiency of the motors.